The composite die steel was prepared by vacuum diffusion welding with the cold work die steel Cr12MoV and the alloy steel 40Cr. The composition, microstructure and property changes of the welded joints were studied experimentally, and the relationship between the microstructure and the property after the final heat treatment was analyzed. Based on the experiment results, the suitable welding and heat treatment process were approached. The results show that the good metallurgical bonding is obtained via welding at 1100oC for 30 minutes with a pressure of 20 MP and not more than 0.01 Pa in vacuum. A number of voids are found in the tensile fracture surfaces when the welding temperature is below 1100oC. The grains of 40Cr steel grow obviously when the welding temperature is higher than 1100oC. After quenching at 1050oC and tempering at 200oC, the better mechanical properties are obtained in Cr12MoV because fine carbides distribute evenly. At the same time, the mechanical properties of 40Cr do not decrease dramatically because even though high temperature quenching brings coarse grain size, the amount of lamellar martensite, which is hard and brittle, decreases a lot and the amount of lath matensite increases. After quenching and tempering treatment, the bending strength of the composite die steel reaches 73% of that of Cr12MoV when treated by conventional processing. In the course of welding, carbon migrates from 40Cr to Cr12MoV, which results in a decrease in hardness at the interface of 40Cr.
Self-leveling mortar was prepared with high alumina cement as dominant binder.Its compressive strength could reach 6.7 MPa when the age was 3 h.Its compressive strength and flexural strength grew to 45.7 MPa and 13.4 MPa respectively,when the age was 28 d.So the mechanical property could meet the requirement of industry surface layer.On the basis of this formulation,dimensional change ratio of early age(0~24 h)and later age(1~28 d) of the self-leveling mortar were both tested with the help of a kind of thin-mortar laser device and the vertical shrinkage mortar tester.Finally the limitation of standard JC/T 985—2005 on the dimensional change ratio of self-leveling mortar was discussed.
Abstract In this work, pitch-based carbon fibers were utilized to reinforce silicon carbide (SiC) composites via reaction melting infiltration (RMI) method by controlling the reaction temperature and resin carbon content. Thermal conductivities and bending strengths of composites obtained under different preparation conditions were characterized by various analytical methods. Results showed the formation of SiC whiskers (SiC w ) during RMI process according to vapor—solid (VS) mechanism. SiC w played an important role in toughening the C pf /SiC composites due to crack bridging, crack deflection, and SiC w pull-out. Increase in reaction temperature during RMI process led to an initial increase in thermal conductivity along in-plane and thickness directions of composites, followed by a decline. At reaction temperature of 1600 °C, thermal conductivities along the in-plane and thickness directions were estimated to be 203.00 and 39.59 W/(m·K), respectively. Under these conditions, bending strength was recorded as 186.15±3.95 MPa. Increase in resin carbon content before RMI process led to the generation of more SiC matrix. Thermal conductivities along in-plane and thickness directions remained stable with desirable values of 175.79 and 38.86 W/(m·K), respectively. By comparison, optimal bending strength improved to 244.62±3.07 MPa. In sum, these findings look promising for future application of pitch-based carbon fibers for reinforcement of SiC ceramic composites.
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